It is known to charge the rechargeable or secondary battery of an electric device, in particular of a household appliance, in different charging steps applying different charging currents. Due to the normal usage behavior, the charging steps regularly differ from standard charging methods, such as cc-cv (constant current-constant voltage) charging methods.
In a first step, the loading occurs with a low current of e.g. 1/10 C. C refers to the (total) capacity of the rechargeable battery, e.g. 1000 mAh. With a charging current of 1 C, the rechargeable battery would be completely charged after one hour. Accordingly, with a capacity of 1000 mAh, charging with 1 C would lead to a current of 1000 mA. The first step of charging is followed by a second step in which the loading occurs with a high current, e.g. a current of about 1 C. This charging is normally performed until a voltage threshold and/or a temperature threshold of the battery is reached. In a third step, the battery is charged with a pulsed charging current.
The voltage threshold, the value of the charging current and the internal resistance of the battery determine the status of charge that can be maximally reached. For Lithium-ion batteries the reachable status of charge with this charging method is usually smaller than with a standard cc-cv charging method.
With deterioration (aging) of the battery its internal resistance is increasing. In consequence, the maximum reachable status of charge is decreasing with the life time of the battery. This means that in particular for Lithium ion batteries less electric charge can be loaded during the charging process though the capacity in the battery would still be available. The reason is that the voltage threshold is reached earlier due to the increased internal resistance. In turn, the runtime of the device or appliance decreases with the lifetime of the battery because the                theoretically available electric capacity of the battery is not used.        
The same is valid at low ambient temperatures as the internal resistance of the battery increases with decreasing temperature of the battery.
In case of a high internal resistance due to low ambient temperature of e.g. 5° C. and a high pulsed charging current in the third charging step, a respective voltage threshold for terminating the charging pulse might be reached even upon switching on the pulsed charging current. As a result, no effective charging occurs in said third charging step.
The same effect leads to higher voltage pulses or jumps upon switching on the current pulses in the third charging step. Further, the frequency of switches the current pulses is increased. This might lead to a worse electromagnetic compatibility (EMC) or a respective technical and cost effort to avoid the deterioration of the electromagnetic compatibility.
Further, if a device or appliance is not removed form the mains or switched on after a charging process a pulsed charging current might be provided with a reduced switching-off voltage threshold in order to just compensate for the consumption of the standby current of the device or appliance and/or the self-discharging of the battery. This leads to retention of the status of charge. Having a high internal resistance due to any of the before mentioned effects, the reduced switching-off voltage might be reached immediately upon switching on the current pulse. In this case the charging of the battery might not compensate for the standby current or the self-discharging any more.
It is accordingly an object of the present invention to improve a charging method in particular for household appliances or electric devices which generally are used only for short periods during one day. In particular, a charging method shall be provided that can also be used for Lithium-ion batteries.